(* ps2-explain.sml
Solution to Problem 2 of Problem Set 2. *)
(* val shortest : (int -> bool) -> str -> str * str
If w is an str of zero's and one's, and there is a prefix x of w
such that f(diff x), then shortest f w returns (x, y), where x is
the shortest such prefix and y is such that x @ y = w. *)
fun shortest f (w : str) : str * str =
let fun short(bs, n, nil) =
if f n then (bs, nil) else raise Fail "shouldn't happen"
| short(bs, n, c :: cs) =
if f n
then (bs, c :: cs)
else short(bs @ [c], n + diffSym c, cs)
in short(nil, 0, w) end
(* val shortestPositive : str -> str * str
If w is an str of zero's and one's, and there is a prefix x of w
such that diff x >= 1, then shortestPositive w returns (x, y),
where x is the shortest such prefix and y is such that x @ y = w. *)
val shortestPositive = shortest(fn n => n >= 1)
(* val shortestNegative : str -> str * str
If w is an str of zero's and one's, and there is a prefix x of w
such that diff x <= ~1, then shortestNegative w returns (x, y),
where x is the shortest such prefix and y is such that x @ y = w. *)
val shortestNegative = shortest(fn n => n <= ~1)
(* val splitPositive : str -> str * str
If w is an str of zero's and one's and diff w >= 1, then
splitPositive w returns a pair (x, y), such that w = x @ [zero] @ y,
diff x = 0 and diff y = diff w - 1. *)
fun splitPositive w : str * str =
let val (u, y) = shortestPositive w (* w = u @ y *)
(* diff u >= 1 and shorter prefixes of w have non-positive diffs *)
val x = Str.allButLast u (* diff x = 0 *)
val b = Str.last u (* isZero b, diff y = diff w - 1 *)
in (x, y) end
(* val explain : str -> expl
If w is in Y, then explain w returns an explanation expl such
that Str.equal(strExplained expl, w). *)
fun explain (w : str) =
if null w
then Rule1
else if isZero(hd w)
then let val t = tl w (* w = [zero] @ t, diff t = ~1 *)
val (u, v) = shortestNegative t (* t = u @ v *)
(* diff u <= ~1 and shorter prefixes of t than u have
non-negative diffs *)
val x = Str.allButLast u (* diff x in {0, 1} *)
val b = Str.last u (* isOne b *)
in if diff x = 0
then (* diff v = 1 *)
let val (y, z) = splitPositive v (* v = y @ [zero] @ z *)
(* diff y = 0, diff z = 0,
w = ([zero] @ x @ [one] @ y @ [zero]) @ z *)
in Rule5(Rule3(explain x, explain y), explain z) end
else (* diff x = 1, diff v = 0, x begins with 0 *)
let val y = tl x (* diff y = 0 *)
(* w = ([zero] @ nil @ [zero] @ y @ [one]) @ v *)
in Rule5(Rule2(Rule1, explain y), explain v) end
end
else (* isOne(hd w) *)
let val t = tl w (* w = [one] @ t, diff t = 2 *)
val (x, u) = splitPositive t (* t = x @ [zero] @ u,
diff x = 0, diff u = 1 *)
val (y, z) = splitPositive u (* u = y @ [zero] @ z,
diff y = 0, diff z = 0 *)
(* w = ([one] @ x @ [zero] @ y @ [zero]) @ z *)
in Rule5(Rule4(explain x, explain y), explain z) end;